Ion exchange strengthening of glasses with lithium vapor

ABSTRACT

Soda-lime glass articles are strengthened by exposure to vapors of certain lithium compounds while heated to or above the annealing point. Exposure at temperatures below the annealing point is avoided, to prevent etching the glass surface. Lithium ions migrate into a zone of the glassy network underlying the exposed surface and partially replace alkali metal ions therein. Upon cooling, the lithium-enriched shell is placed in compression about the glass article. Strength improvements are achieved in short periods of time.

Inventor Francis .1. Shoneharger Lancaster, Ohio Appl. No. 9,43%

Filed Dec. 111, 11967 Patented Oct. 26, 197i Assignee Anchor HockingCorporation Lancaster, Ohio HON EXCHANGE STRENGTHENING 0F GLASSESWl'ilil LITHIUM VAPOR l 1 Claims, 2 Drawing Figs.

IILS. Cl 65/30,

65/60, 117/106, 117/124 lint. Cl. C03c 211/00 Fielld all Search 65/30,60,

[56] References Cited UNITED STATES PATENTS 2,779,136 1/1957 l-lood etal. 65/30 FOREIGN PATENTS 748,820 6/1962 Great Britain 65/30 irimaryExaminer-S. Leon Bashore Assistant Examiner-John H. HarmanAttorney-Wood, Herron & Evans Heat glass article containing replaceableNd ions to temperature at which its viscosity is 10 IO poises Expose tovapors at LiBr for period between i minute and lhour white at said!temperature Terminate exposure to vapor rtict More onnalt intPATENTEDUET 26 I9?! 3,615,319

Heat glass article containing ceabte No ions to temperotur -ch itsviscosity is I0 poises se to vapors of LiBr for 0d between I 'ute andlhour while at temperature Terminate exposure to vapor Cool articlebelow annealing point INVENTOR.

A TTOENE Y5 ION IEX'CHANGE STRENGTHENING F GLASSES WITH LITHIUM VAPORThis invention relates to a simplified method of strengthening glassarticles by forming a compression shell in situ around the body of thearticle.

Various methods are known for strengthening glass articles by impartingor fonning a compression layer on external surface areas of the article.One such method is tempering, wherein the heated article is quicklychilled so that the outside layer of the article is cooled and shrinks"before the interior and is thereby placed in compression as the insidecools.

Another strengthening method involves the substitution or exchange, in ashallow surface layer, of smaller ions of an alkali metal from anexternal source, for larger alkali metal ions in the glass. In contrastto thermal tempering where the surface layer is placed in compression bydifferential cooling rates, in the ion exchange technique the surfacelayer is placed in compression around the interior of the body by reasonof the fact that its composition is changed from that of the interiorand its thermal contraction on cooling is less than the interior.

Stookey et al. U.S. Pat. No. 2,779,136 shows a prior method ofstrengthening by ion exchange. In that method the article to bestrengthened is treated with a molten bath containing a lithium salt,and the smaller lithium ions are exchanged for the .larger sodium ionsin the glass surface.

Although high strengths can be achieved by the method of US. Pat. No.2,779,136, it is a difficult if not impractical technique to apply inlarge-scale commercial practice because of the fact that it requiresphysically immersing the article in the molten salt bath. The cost oflithium salt is relatively high for the large baths required. Moreoverthe accumulation of sodium ions in the salt bath quickly reduces itsutility, thereby requiring frequent replacement of the bath. Therequired immersion necessitates handling of each article at hightemperature, and does not lend itself readily to treatment of hollowarticles. The residue which adheres to the article upon its removal fromthe bath makes cleaning more difficult.

Perhaps the greatest difficulty with the previous lithium salt bathmethod is that the articles so treated are frequently clouded or frostedby contact with the liquid lithium compound, so that the article losesits clarity, thus rendering it useless for many purposes unless it isgiven special further polishmg.

This invention is predicated upon the determination that theobjectionable crazing or etching which was often incidental to the saltbath method of the prior art, can be avoided and other disadvantages ofthat method are obviated while still obtaining good strength increasesin short treating times, if the article to be treated is exposed only tothe vapors of certain lithium compounds, out of contact with any moltensalt bath, and provided the vapor exposure is carried out only attemperatures above the annealing point of the glass' In general, thelithium compounds utilized have very low vapor pressures and would notbe expected to produce any effective strengthening by reason of the factthat the concentration of lithium ions in the vapor state adjacent thesurface of the glass article is extremely small in relation to the levelof concentration provided by a molten salt bath.

Nonetheless, I have found that it is possible to achieve strengthimprovements of as much as 150 percent, even -by vapor treatments asshort as minutes. At the same time the clarity of the glass ispreserved, and the clouding often caused by salt bath treatment isavoided.

The present invention can be used to treat articles of any glasscomposition which contains replaceable sodium and/or potassium ions. Inbroad terms, this includes glass compositions having about 725 totalpercent by weight of Na O and K 0 and about 45-80 percent $0,. Theremainder may be constituted of other conventional glass-makingcompounds including CaO, A1 0 MgO, ZnO, Li O, BaO, etc. Included in thisgroup are the common soda-lime glasses of which containers and windowsare ordinarily made and which comprise about 65-74 percent SiO 14-17percent Na O, 7-l2 percent 2 (MgO+CaO) and up to 3 percent M O,.Potassium oxide-containing glasses are much less important commerciallythan soda-lime glasses, but can also be treated by the method of thisinvention. The presence of more than about 5 percent 5,0; in thecomposition tends to reduce the replaceability of t the sodium orpotassium which is present.

The article to be treated can be of virtually any form, including bothnarrow-necked and wide-necked containers, sheet glass, molded or blownshapes, fibers and so on.

The lithium compounds which are useful herein are those which willvolatilize without substantial decomposition or adverse reaction undertreating conditions, and which can supply lithium ions (Li-l to theglass surface. These include the halides, that is, lithium bromide,lithium chloride, lithium iodide and lithium fluoride. Other salts suchas lithium sulfate can also be used, alone or in mixture with a lithiumhalide. Lithium amide, UNI-I is useful in the absence of water vapor andoxygen but tends to form lithium hydroxide, LiOH, in the presence ofwater vapor, which cannot be used in the method of this invention. Themelting points and comparative vapor pressures of some of thesecompounds are as follows:

Lithium vapors can be generated directly in the treating chamber, forexample from a crucible or shallow pan of the powdered material.Alternatively the vapors can be generated in a separate chamber andducted into the treating chamber. To establish a more uniform vaporconcentration it may be useful to employ a circulating fan. Where theinside of a hollow object is to be treated, means such as a nozzle canadvantageously be used to establish more adequate exposure of suchsurfaces.

Broadly stated, the method according to this invention comprises formingan article of a glass composition containing replaceable sodium orpotassium ions, heating the article to a temperature at or preferablyabove its annealing point, exposing it to vapors of the lithium compoundfor a period of time sufficient to permit a portion of the sodium andpotassium ions adjacent the surface of the glass article to be replacedby lithium ions from the vapor, the exposure being carried outessentially only during the time when the article is above its annealingpoint, then cooling the article, the cooling from the annealing point tobelow about 800 F. being carried out in the absence of such vapor. Thepeak temperature of the heating cycle should not be so, high as to causeundesirable deformation of the article. i

I have found that it is important that the article he substantiallyexposed to lithium vapors only while heated to temperatures at or aboveits annealing point. That is, the article should be heated to and cooledfrom the treating temperature in the substantial absence of lithiumvapors. Exposure of the article to lithium vapors while heated, butbelow the annealing point, tends to cause surface clouding. This effectis especially severe at temperatures between about F. and the annealingpoint. However, such surface clouding or loss of transparency does nottake place if the article is exposed to vapor of the lithium compoundonly at temperatures at or above the annealing point.

Under such proper exposure conditions, the products treated by thismethod display essentially the same transparency after strengthening asthey displayed before it, and no clouding of the surface is manifest.

I have further found that exposure times between 1 minute and 1 hour attemperatures corresponding to glass viscosities of about lo 10 poiscsare the most useful.

The term annealing point, as used herein, has been defined by theAmerican Society for Testing Materials as that temperature at which theglass article has a viscosity of IO poises. The annealing point can bedetermined by the method of ASTM C336-54l, as described in A.S.T.M.Standards on Glass and Glass Products, Methods of TestingSpecifications, th Ed., Dec. 1962.

If a glass article containing replaceable sodium or potassium is heatedin the presence of lithium vapors of the type described, lithium ionswill migrate from the surface of the glass into the interior, displacingor replacing sodium and/or potassium ions therein. Displacement ofsodium ions, for example, by this process can readily be detected by theformation of a film of the corresponding sodium salt on the surface ofthe article. The presence of exchanged lithium can be confirmed bychemical analyses performed on glass dissolved from a thin layer of thetreated surface.

The following glass compositions, in parts by weight as analyzed, areillustrative of various types of glasses which can be used in carryingout the new method.

COMPOSITION Oxide A B C SiO, 67.9 73.0 55.2 Na,0 15.8 HA 18.3 20 0.5 0.14.l 20, 2.9 L4 2i .6

Fe,0, 0.04 0.03 0.07 BaO 2.1

MgO 3.8 4.3 0.09 Li,0 trace TiO, 0.02

MnO 0.003

Annealing point, F. 972 1,007 1,080

Expressed as S Compositions A and B are typical soda-lime types ofglasses while composition C is a high sodium-aluminum silicate glass.

In the drawing, FIG. 1 is a flow sheet showing a preferred method ofcarrying out the new method and FIG. 2 is a diagrammatic section ofapparatus useful for carrying out the method on a batch basis.

EXAMPLE 1 Glass rods 1 of composition A, of very uniform cross section,were placed on a refractory support 2 in a horizontally mounted tube 3as shown in FIG. 2. The glass rods 1 were positioned at different axialpositions along the length of the tube, as shown. The tube 3 was heatedexternally by resistive coils 4 along its length. The ends of tube 3were closed by refractory plugs 5 and stoppers 6. The tube 3 and rods 1therein were heated in air to approximately 1,100 E, corresponding to aviscosity of the glass of composition A of about poises. A small hole 7in each stopper 6 permitted atmospheric pressure to be maintained in thetube 3. A porcelain boat 8 containing powdered lithium bromide (LiBr) 9was separately preheated to l,l00 F. and when it had reached thattemperature, was placed into one end of the tube 3. The LiBr 9volatilized from the boat 8 and penneated the interior of the tube 3.Molecules of vaporized LiBr came into contact with the surface of therods 1 and a portion of the lithium, be-

lieved to be in the ionic state, migrated from the surface into theglass network in a zone underlying the surface, displacing sodiumtherein;

The furnace was held at 1,l00 F. for minutes with the source of LiBr inplace, after which the boat 8 was removed. The rods were cooled to below700 F over a period of about minutes. A white powdery film was visibleon the surface of the glass rods upon their removal from the treatingfurnace.

The film was completely removable by rinsing with water, and uponanalysis was identified as NaBr. This is an indication that lithium ionsdiffused into the surface zone and replaced a proportion of the sodiumtherein and that the displaced sodium ions recombined with bromideanions on the surface of the glass to form the sodium bromide film. Theglass rods were clear (uncolored) and the lithium enrichment of thesurface imparted no visually detectable color, either as viewed in thedirection perpendicular to the surface or parallel to the surface.

The surface of the rods was not etched or crazed and displayed noclouding of the type frequently found on the surface of rods which aretreated in molten lithium salt. For example, if rods of this samecomposition are treated in'a molten lithium bromide bath for 15 minutesat l,l00 F., etching is manifest which would render the rods useless formost purposes,

without special polishing. Even the presence on the article duringcooling in air of a lithium salt film from a molten treating bath cancause etching.

After cooling and rinsing, the rods were tested for abraded strength.This test procedure was carried out by placing the rods in a jarcontaining ISO-mesh silicon carbide. The jar was rotated at r.p.m. on aball mill rack for 5 minutes. The rods were stressed by applying load atthe center of a 2-inch span and increasing the load at a rate of about 8pounds per second until fracture occurred. The approximate strengths ofthe rods in p.s.i., as calculated from their cross section, rangedbetween 26,100 and 51,300 p.s.i., and varied with the LiBr concentrationalong the tube.

The abraded strength of untreated glass rods of the same composition wasabout 20,000 p.s.i. Thus, the LiBr treatment effected strength increasesof about 30 to percent as compared to the controls. Rods given a similarthermal cycle in the absence of LiBr vapor quickly abrade to theirinitial strength.

EXAMPLE 2 Glass rods of composition A, approximately 3/16 inch X 4%inches, were placed in a stainless steel vessel or chamber about 8inches high, 10 inches wide, and ll inches long. Powdered LiCl wasspread in a uniform layer about 56-inch deep over the bottom of thechamber. The glass rods to be treated were supported on refractory barsin the chamber. The chamber was placed in a gas fired circulatingatmosphere furnace. Circulation rapidly swept the vapor from thechamber, so that essentially no lithium vapor collected in the chamberuntil a cover or lid was placed on it.

The furnace was heated from room temperature to 1,130 F and the coverwas then placed on the chamber to concentrate the LiCl vapor in it.After a 1 hour exposure period the cover was removed from the chamber toeffectively tenninate the exposure, and the furnace was permitted tocool at its natural rate over a period of about 1 hour, at which timethe rods were removed. After removal of a powdery NaCl film, the rodswere found not to be clouded, etched or crazed by their exposure to thevapor. The average strength of the rods so treated was found to be about29,000 p.s.i. Longer abrading of both the controls and the treated rodscauses a much greater differential between the comparative strengths.

The calculated vapor pressures of the lithium halides at 1,100 F. arevery low, as apparent from the following tabulation:

LiBr 0.063 mm Hg Lil 0.058 mm Hg LiCl 0.026 mm Hg LiF 0.00003 mm HgEXAMPLE 3 Glass rods of composition A were treated with LiCl in thechamber specified in example 2. The rods were heated to um i 1,100 F.,and four crucibles containing powdered LiCl were then placed in thechamber and the lid was closed. The rods were exposed for 1 hour afterwhich the crucibles were removed and the rods cooled out of substantialcontact with LiCl.

The rods so treated had a visible, white, easily removable sodiumchloride film on the surface, but after removal of the film weretransparent and were not etched or crazed by the vapor treatment. Theyhad an average abraded strength of 24,800 p.s.i., as compared to 18,600p.s.i. for an untreated standard at the same abrasion conditions.

EXAMPLE 4 Glass rods of composition A were placed in a chamber about 4%inches high, 10 inches wide and l 1 inches long. The rods were supportedon refractory bars about 1% inches above the bottom of the chamber. Twocrucibles filled with powdered lithium fluoride were placed in thetreating chamber. The exposed surface of the LiF in the crucibles was acircle about 1% inches in diameter and was about one-half inch from thecrucible rim. The chamber was closed and placed in a furnace with a veryrapid heat-up rate. With both the rods and crucibles in place, thefurnace was heated to about l,l F. at rate of about 3,000 F. per hour,and was held at 1,100 F. for 1 hour. The rods so treated had an averagestrength of 25,000 p.s.i. compared to the standard rods with averagestrength of 20,000 p.s.i., and were free from clouding.

Examples 2-4 demonstrate that where the heat-up rate is very rapid sothat there is only brief exposure of the articles to the lithium vaporat temperatures above about 800 F. but below the annealing point, orwhere the concentration of the vapor is very low during that range ofheating, such insubstantial exposure, even though it is below theannealing point, does not cause clouding. However, if the rods wereexposed to more concentrated vapors over a gradual heat-up, the dangerof etching or clouding is significant.

EXAMPLE 5 Lithium amide, LiNl-l,, has a high vapor pressure but tends toform lithium hydroxide and ammonia in the presence of water vapor:

Under such conditions theaTnide is not generally useful according to themethod of this invention. However, I have found that the amide can beused in the substantial absence of water. This is demonstrated by a testin which rods of composition A were placed in a tube furnace and exposedto LiHG vapors at 1,100 F. for minutes while dry nitrogen was passedthrough the tube to sweep out any water vapor. Upon cooling, the rods sotreated were unciouded and dis played 'an average tensile strength ofabout 33,500 p.s.i. The rods were free from etching, although they had anoticeable brownish coloration.

EXAMPLE 6 Like the lithium halides, lithium sulfate can be used in anordinary atmosphere, and it shows no tendency to color the rods.Moreover, I have found that it effects unusually large strengthimprovements in short treating times.

Rods of composition A were placed in a tube furnace of the typedescribed in example 1, and were heated to 1,100 F. in air. When thefurnace reached l,l00 F., a boat containing H 50 preheated to l,l00 F.,was placed in the furnace. Temperature was held at 1,100 F. for 10minutes, after which the boat was removed and the furnace was cooled.The rods displayed strengths of 42,300 p.s.i. and were free fromstaining or etching.

EXAMPLE 7 One convenient means of providing a large surface area of thelithium source is to saturate it into a porous refractory material. Inthis example, a porous insulating fire brick, 4% inches X 9 inches X 1inch standing on the 9 inches X 1 inch face, was saturated with grams oflithium bromide. The brick and LiBr were individually preheated, and theLiBr was poured onto the brick while both were at the same temperature.The brick may then be cooled to room temperature before use.

Nonreturnable, clear beverage bottles of composition 18 were heated at1,100 P. and the brick saturated with lithium bromide was placed in afurnace for 15 minutes at l,l00 F. The furnace was heated on all sixsides to maintain even temperature conditions. The brick was thenremoved and the bottles were cooled in the furnace to room temperature.

Without treatment, these bottles had average internal breakage pressuresof 366 p.s.i. as measured on a standard intemal pressure tester. Aftertreatment, two of the four bottles had average strengths in excess ofthe maximum 550 p.s.i. of which the tester was capable, a third broke at550 p.s.i., and the fourth broke at the base contact point under apressure of 375 p.s.i.

EXAMPLE 8 Bottles of composition B were treated according to the methoddescribed in example 7, except that the furnace was held at about the1,007 F. annealing point of the bottles for 60 minutes. The averagestrength of these bottles was 425 p.s.i.

By way of comparison, when similar bottles were treated at 950 F. (belowthe annealing point) for a period of i 45 minutes, their strength wasactually reduced, to a level lower than that of the untreated bottles.

EXAMPLE 9 Soda-lime rods of composition A were heated to l, 1 00 F. Apreheated brick saturated with LiBr' was then placed in the furnace andthe rods were exposed to the vapor for 15 minutes, at the end of whichthe brick was removed and the furnace cooled to room temperature. Theaverage strength of the rods was 35,700 p.s.i.

EXAMPLE 10 The freedom from etching of glass articles treated inaccordance with the method of this invention, in comparison to treatmentat the same conditions with a molten salt bath, is illustrated by thefollowing comparison: One end of apiece of ordinary soda-lime windowglass was placed in a melt of LiBr at 1,150 F. for 30 minutes, thenremoved and cooled. The other end of the sample was out of contact withthe molten salt, but was exposed to LiBr vaporized from the melt. Theportion of the sample which had been in the melt was severely etched,but the upper portion was clear.

In articles treated by the method of this invention, the lithiumion-enriched surface layer is in compression. This can be ascertained byviewing a thin fractured sample edg'ewise in polarized light betweencrossed nicols in a microscope, according to known technique.

The presence of a visible white sodium or potassium salt film on theexposed surface of the treated articles can be taken as one convenientindication that the heating cycle has caused the lithium vapor todisplace other alkali ions in the glass.

It is important, from the standpoint of achieving useful strengthincreases at rapid rates, that the lithium vapor concentration be ashigh as possible. To that end, subatmospheric pressures in the treatingchamber can be used to increase the lithium ion concentration.Decreasing the distance from the emitting source to the ware also servesto enrich the vapor concentration at the glass surface.

The peak temperature at which any specific glass article can be mosteffectively treated depends upon the shape of the article. In generalthe article should not be treated at temperatures at which undesirabledeformation or sagging out of the proper final shape occurs.Temperatures which are suitable for articles of one particular shape maytend to cause undesirable sagging of larger, more complex, or moredelicate shapes of the same glass composition. On the other hand,treatment at higher temperatures can be used for shapes wheredeformation is not a critical operating restriction, for example withsheet glass, or where deformation is needed to cause the article to saginto a desired final shape. Ordinarily treatment at viscosities in therange of lto poises is the more suitable, and treatment at glassviscosities of about l0 to 10''- poises is particularly useful becauseit appears to effect ion exchange in a relatively sharply defined layer.For soda-lime glasses, temperatures of about 25-l50 F. above theannealing point are suitable.

While the foregoing examples illustrate the preferred practice of thisinvention, it should be understood that I do not intend to be solimited, and that the invention also includes other embodiments andmodifications falling within the scope of the claims which follow.

I claim:

1. A method of increasing the strength of a glass article made from acomposition including 7-25 percent (Na O+K,), 45-80 percent SiO,, andnot more than about 5 percent B 0 said method comprising,

first heating the article to a temperature not lower than its annealingpoint, but below the temperature at which the article sags out ofdesired shape, in the absence of substantial amounts of lithium treatingvapor,

subsequently exposing at least a portion of the surface of said article,while heated to said temperature, to vapor of at least one lithiumcompound selected from the class consisting of LiBr, LiCl, LiF, Lil,LiNH, in the absence of water, and Li,SO,, and continuing said exposurefor a period of time sufficient to replace a portion of the alkali metalions with lithium ions from said vapor,

and cooling said article below the annealing point the exposure tolithium vapor being prevented during cooling while said article isbetween the annealing point and about 800 F.

2. The method of claim 1 wherein said exposure is for a period between 1minute and 1 hour.

3. The method of claim 1 wherein said exposure is carried out by placinga supply of said compound adjacent to said article in a furnace.

4. The method of claim 1 wherein said exposure is carried out attemperatures corresponding to glass viscosities of about l0- to 10poises.

5. The method of claim 1 wherein said exposure is carried out attemperatures corresponding to glass viscosities of lo to 10 poises.

6. The method of claim 1 wherein said composition is a soda-lime glasscomposition comprising about 65-74 percent SiO,, 14-17 percent Na,0,7-l2 percent (MgO+CaO), and up to 3 percent A50 7. The method of claim 1wherein said exposure is carried out at a temperature of 25-l 50 F.above the annealing point of said article.

8. A method for making a strengthened glass article comprising,

first, melting a batch of glass-making ingredients so as to provide aglass batch having on an oxide basis about 65-74 percent SiO,, 14-17percent N3 0, 7-l2 percent (MgO-iCaO) and up to 3 percent AI,O,

forming at least one article from said glass batch,

introducing said article into a treating chamber and heating it thereinto a temperature above its annealing point but below the temperature atwhich the article sags out of desired shape, in the absence ofsubstantial amounts of lithium treating vapor,

subsequently introducing into said chamber, while said article is atsaid temperature, a vapor of at least one lithium compound selected fromthe class consisting of LiBr, LiCl, LiF, Lil, LiNH, in the absence ofwater, and 11,80

and exposing at least a portion of the surface of said article to saidvapor in said chamber at said temperature for a period of time between 1minute and 1 hour sufiicient to replace a portion of the alkali metalions with lithium 1011s,

terminating the exposure of said article to said vapor,

then cooling said article from said temperature to room temperature inthe absence of vapor.

9. The method of claim 8 wherein a supply of said compound is placed inthe chamber adjacent said article.

10. The method of claim 9 wherein said supply is removed from saidchamber before said article is cooled from said temperature.

11. The method of claim 9 wherein said supply is preheated to thetemperature at which said article is to be exposed, before said sourceis placed in said chamber.

i 1'! 1R I i UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTIONPatent No- 3,615,319 Dated October 26, 1971 Inventor(s) Francis JosephShonebarger It is certified that error appears in the above-identifiedpatent and that said Letters Patent are hereby corrected as shown below:

Column 2, line 63, "80 F." should be --800 F.-

Column 3, line 5, C336-54I" should be --C336S4T-- Column 3, CompositionTable, separating line should be under column titles Column 3,Composition Table line 3, "20" should, be --K O-- Column 3, CompositionTable line 4, "20 should be --A1 0 Column 5, line 51, "LiHG should be--LiNH Column 7, line 22, (NA 0 K should be (NA 0 x 0) Column 8, line 2,"10 sh ld b --1 Signed and sealed this 18th day of April 1972.

(SEAL) Attest:

EDWARD M.FLETCHER,JR. ROBERT GOTTSCHALK Attesting Officer CommissionerofPatents RM 90-1050 (IO-59} USCOMM-DC 60376-5 69 ".5. GOVlEmnzm Flm'rnmMn". M.

2. The method of claim 1 wherein said exposure is for a period between 1minute and 1 hour.
 3. The method of claim 1 wherein said exposure iscarried out by placing a supply of said compound adjacent to saidarticle in a furnace.
 4. The method of claim 1 wherein said exposure iscarried out at temperatures corresponding to glass viscosities of about1010.0 to 1012.6 poises.
 5. The method of claim 1 wherein said exposureis carried out at temperatures corresponding to glass viscosities of1010.6 to 1011.7 poises.
 6. The method of claim 1 wherein saidcomposition is a soda-lime glass composition comprising about 65-74percent SiO2, 14-17 percent Na2O, 7-12 percent (MgO+CaO), and up to 3percent Al2O3.
 7. The method of claim 1 wherein said exposure is carriedout at a temperature of 25*-150* F. above the annealing point of saidarticle.
 8. A method for making a strengthened glass article comprising,first, melting a batch of glass-making ingredients so as to provide aglass batch having on an oxide basis about 65-74 percent SiO2, 14-17percent Na2O, 7-12 percent (MgO+CaO) and up to 3 percent Al2O3, formingat least one article from said glass batch, introducing said articleinto a treating chamber and heating it therein to a temperature aboveits annealing point but below the temperature at which the article sagsout of desired shape, in the absence of substantial amounts of lithiumtreating vapor, subsequently introducing into said chamber, while saidarticle is at said temperature, a vapor of at least one lithium compoundselected from the class consisting of LiBr, LiCl, LiF, LiI, LiNH2 in theabsence of water, and Li2SO4, and exposing at least a portion of thesurface of said article to said vapor in said chamber at saidtemperature for a period of time between 1 minute and 1 hour sufficientto replace a portion of the alkali metal ions with lithium ions,terminating the exposure of said article to said vapor, then coolingsaid article from said temperature to room temperature in the absence ofvapor.
 9. The method of claim 8 wherein a supply of said compound isplaced in the chamber adjacent said article.
 10. The method of claim 9wherein said supply is removed from said chamber before said article iscooled from said temperature.
 11. The method of claim 9 wherein saidsupply is preheated to the temperature at which said article is to beexposed, before said source is placed in said chamber.